Acoustic vibrations or “noise” perceptible by human beings is a continuing problem in systems where air circulation occurs at moderate to relatively high velocities. For example, forced flow air conditioning systems for commercial and residential applications, of necessity, circulate air by mechanical fans or blowers through enclosures, ductwork and related structures. Human audible noise generated by this air flow is desirably reduced as much as possible, but the practical requirements of air flow systems of the general type mentioned above require air flow velocities and structural features which cause acoustic vibrations perceptible to the human ear. For example, structures such as fan guards or grilles placed over ductwork outlets of various types and over air flow outlets of enclosures for heat exchangers and so-called condenser units in residential and commercial air conditioning systems have been determined to be a source of humanly perceptible noise.
It has been determined that fan guards and similar grille type protective structures associated with forced flow-type air conditioning systems may generate at least some noise as a result of vortex shedding from the downstream side of such structures at certain air flow velocities. Well-known von Karman vortex streets may form at certain air velocities required in air conditioning systems having forced air flow over heat exchangers and for general circulation purposes.
One solution to the problem of von Karman vortex shedding from structures, such as smokestacks and pipelines, is the provision of helical strakes or fins mounted on the exterior of the cylindrical stack or pipeline structure. Although this technique is successful in suppressing formation of von Karman vortex streets, the provision of helical strakes or similar windings in structures associated with forced air flow type air conditioning systems may be somewhat impractical. U.S. Pat. No. 6,470,700 to Qiu, et al. discloses a grille or guard for an air conditioning unit wherein the elongated rod-like members forming the guard are wrapped with wire in a spiral fashion to emulate the wellknown anti-vortex strakes provided on smokestacks, pipelines and similar structures. However, as mentioned above, wrapping the rod-like members of a fan guard or the like with wire poses several problems including increased manufacturing costs, difficulty in cleaning the guard, increased aerodynamic drag and the chance of the wires becoming broken and interfering with operation of equipment placed adjacent to guards, such as an axial flow fan, for example.
Accordingly, there has been a continuing need for further improvements in noise reduction associated with forced flow air handling systems, including forced flow air conditioning systems and the like. It is to these ends that the present invention has been developed.